Abstract

Introduced species are having major impacts in terrestrial, freshwater and marine ecosystems worldwide.It is increasingly recognised that effects of multiplespecies often cannot be predicted from the effect of eachspecies alone, due to complex interactions, but mostinvestigations of invasion impacts have examined onlyone non-native species at a time and have not addressedthe interactive effects of multiple species. We conducteda field experiment to compare the individual and combinedeffects of two introduced marine predators, thenorthern Pacific seastar Asterias amurensis and theEuropean green crab Carcinus maenas, on a soft-sedimentinvertebrate assemblage in Tasmania. Spatialoverlap in the distribution of these invaders is justbeginning in Tasmania, and appears imminent as their respective ranges expand, suggesting a strong overlap infood resources will result from the shared proclivity forbivalve prey. A. amurensis and C. maenas provide goodmodels to test the interaction between multiple introducedpredators, because they leave clear predator-specific traces of their predatory activity for a number ofcommon prey taxa (bivalves and gastropods). Ourexperiments demonstrate that both predators had amajor effect on the abundance of bivalves, reducingpopulations of the commercial bivalves Fulvia tenuicostataand Katelysia rhytiphora. The interaction betweenC. maenas and A. amurensis appears to be one of resourcecompetition, resulting in partitioning of bivalvesaccording to size between predators, with A. amurensisconsuming the large and C. maenas the small bivalves.At a large spatial scale, we predict that the combinedeffect on bivalves may be greater than that due to eachpredator alone simply because their combined distributionis likely to cover a broader range of habitats. At asmaller scale, in the shallow subtidal, where spatialoverlap is expected to be most extensive, our resultsindicate the individual effects of each predator are likelyto be modified in the presence of the other as densitiesincrease. These results further highlight the need toconsider the interactive effects of introduced species,especially with continued increases in the number ofestablished invasions.